Pyrolytic boron nitride is formed by chemical vapor deposition using a process described in U.S. Pat. No. 3,182,006, the disclosure of which is herein incorporated by reference. The process involves introducing vapors of ammonia and a gaseous boron halide such as boron trichloride (BCl3), in a suitable ratio into a heated furnace reactor causing boron nitride to be deposited on the surface of an appropriate substrate such as graphite. The boron nitride is deposited in layers, and may be separated from the substrate to form a freestanding structure of pyrolytic boron nitride. Pyrolytic boron nitride is an anisotropic material having an hexagonal crystal lattice with properties perpendicular to the basal plane, known as the “c-plane,” which are significantly different from its properties parallel to the plane, known as the “a-plane.” Because of the high degree of anisotropy, the mechanical strength of the freestanding pyrolytic boron nitride (hereafter “PBN”) structure is weak in the perpendicular direction which is the direction of PBN layer growth.
Crucibles of PBN are used commercially to melt compounds at elevated temperatures. For example, in the production of semiconductors, crucible of PBN are used to grow GaAs crystals. A PBN crucible is, however, subject to fracture from a build up of stress. For example, when using the Liquid Encapsulated Czochralski method to produce single crystals of GaAs, fracture of the PBN crucible can occur after the crystal is grown and a boric oxide glass (B203), which is used to cover part of the molten mass, freezes in the crucible. On freezing the boric oxide (B203) glass shrinks more than the crucible. The resultant shrinkage mismatch generates stresses within the PBN crucible that can cause it to fracture. Similarly boric oxide is often used as an encapsulant for GaAs in the vertical gradient freeze (VGF) method. The bond between the boric oxide and the PBN surface and the shrinkage mismatch of the boric oxide and PBN on cooling causes a radial tensile stress (perpendicular to the layers), and a compressive stress (parallel to the layers) in the PBN. This causes large stresses to develop in the PBN which, in turn, can cause delamination i.e. peeling to occur or fracture.
Ideally, the PBN should peel away in thin layers of controlled thickness, providing an improved and predictable service life which would also eliminate the risk of catastrophic failure. In the past, there was no predictability to the number of layers that would peel off. For a crucible to have a long service life, it is desirable to control peeling and to preferably limit peeling to a single selected PBN layer of controlled thickness for each crystal growth run. Moreover, the single selected layer should peel uniformly from the body of the crucible when the boric oxide glass is withdrawn.